Q151 / 160 · AP EAMCET 2021 (19 Aug Shift 2) · Physics

In a co-axial, straight cable, the central conductor and the outer conductor carry equal currents in opposite directions. The magnetic field is zero

Correct answer
A
outside the cable
B
inside the inner conductor
C
inside the outer conductor
D
in between the two conductors

Solution

Since the current carried by the outer and inner are equal and in opposite direction, magnetic field produced by the cylinders at a point P(distance r from central axis) is

$\Rightarrow {\vec{B}}_{outer} = \frac{μ_{o}}{4 π} i \frac{\vec{d l} \times \vec{r}}{r^{3}}$

$\Rightarrow {\vec{B}}_{inner} = - \frac{μ_{o}}{4 π} i \frac{\vec{d l} \times \vec{r}}{r^{3}}$

$\Rightarrow \vec{B} = {\vec{B}}_{outer} + {\vec{B}}_{inner} = 0$

Hence, the correct answer is option: 'A'.

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